[USACO13FEB] Tractor

题目链接

大家的 Blog 里面都是做过的题目，只有我的里面什么都没有

那我也开始写一点吧

刷着刷着 DP 不知怎的就出来一道这个题……用时 2 hours 后怒而删两个文件重构……

然后过了……过了……

至今不知道前两遍错在哪里……

Code - first time：连通块染色 +建图，然后二分，广搜判断

Code - second time：连通块染色 +建图，并查集维护连通块大小，做伪 Kruskal

Code - third time：二分，广搜判断

 1 #include <queue>
 2 #include <cstdio>
 3 #include <cctype>
 4 #include <cstring>
 5 #include <iostream>
 6 #include <algorithm>
 7 using namespace std;
 8 
 9 const int maxn = 500 + 10;
10 const int maxk = 500 * 500 + 10;
11 int n, h[maxn][maxn], vis[maxn][maxn], flag;
12 int f[4][2] = { 0, 1, 1, 0, -1, 0, 0, -1 };
13 
14 inline int Abs(int x) { return x > 0 ? x : -x; }
15 
16 inline bool Breath_fs(int sx, int sy, int c) {
17   queue<pair<int, int> > q;
18   q.push(make_pair(sx, sy)), vis[sx][sy] = 1;
19   int cnt = 1;
20   while( !q.empty() ) {
21     int x = q.front().first, y = q.front().second;
22     for(int i = 0; i < 4; ++i) {
23       int tx = x + f[i][0], ty = y + f[i][1];
24       if( tx < 1 || tx > n || ty < 1 || ty > n ) continue;
25       if( Abs(h[x][y] - h[tx][ty]) > c || vis[tx][ty] ) continue;
26       ++cnt, vis[tx][ty] = 1, q.push(make_pair(tx, ty));
27     }
28     q.pop();
29   }
30   return cnt + 1 > ((n * n) >> 1) + ((n * n) & 1);
31 }
32 
33 inline bool Check(int x) {
34   memset(vis, 0, sizeof vis), flag = 0;
35   for(int i = 1; i <= n; ++i) {
36     for(int j = 1; (!flag && j <= n); ++j)
37       if( !vis[i][j] ) flag = flag | Breath_fs(i, j, x);
38     if( flag ) break;
39   }
40   return flag;
41 }
42 
43 int main(int argc, char const *argv[])
44 {
45   freopen("nanjolno.in", "r", stdin);
46   freopen("nanjolno.out", "w", stdout);
47 
48   scanf("%d", &n);
49   for(int i = 1; i <= n; ++i) for(int j = 1; j <= n; ++j) scanf("%d", &h[i][j]);
50   int lef = 0, righ = 1000000, mid = (lef + righ) >> 1, ans = 0;
51   for( ; lef <= righ; mid = (lef + righ) >> 1)
52     Check(mid) ? ans = mid, righ = mid - 1 : lef = mid + 1;
53   printf("%d\n", ans);
54 
55   fclose(stdin), fclose(stdout);
56   return 0;
57 }

—— 晨意微寒秋渐深，侧畔无事俏佳人。